The invention relates to an apparatus for the non-destructive testing of samples. The samples can in this respect also be formed from critical materials which change their properties during manufacture, for example on a hardening or solidification.
The invention can furthermore be used to characterize other materials which pass through a phase transition or which change as a consequence of other mechanical, thermal, chemical or biological processes. Use is, however, also possible on solid bodies, already solidified or hardened samples.
The exact course of the solidification process or of the hardening process of materials such as cement, mortar, plaster and concrete plays a decisive role in deciding whether the fully hardened construction element has reached its planned mechanical properties, in particular its strength, or not. Since the hardening process depends on a number of parameters such as on the capacities of the concrete mixer, on the properties of the starting products (cement, aggregates, additives), on the water-to-cement ratio, on the environmental temperature, on the humidity, on the incident solar radiation, etc., the conditions for the hardening can never be kept perfectly constant, which has the consequence of fluctuating material qualities. To determine the actual course of the solidification process or of the hardening process and the quality of the respective hardened materials, inexpensive and portable test processes are therefore required which measure physical parameters, which have a sufficiently good correlation to the mechanical material properties, over the total duration of the hardening. These parameters can e.g. be the speed and the damping of ultrasonic pressure waves and shear waves or the moisture content.
A simple, widespread method in the construction industry is the removal of small core samples which are subsequently subjected to further tests such as tensile tests and bending tests in the laboratory. This method cannot be carried out in a non-destructive manner. The core samples can only be removed at a few points of the construction element. The drilled holes have to be filled again after thee removal. The core samples can moreover only be removed and examined in a largely hardened state. The important early stages of the hardening process and its total course remain out of consideration.
A rebound hammer, also called a Schmidt hammer, is suitable for the non-destructive testing of the compressive strength of concrete. It has the disadvantage that measurements are only possible at points of the surface and it can only be sensibly used in the late stage of the hardening process.
Non-destructive test processes which can also be used in early stages of the hardening process and can thus monitor the whole hardening course are usually based on ultrasound technology. In this respect, ultrasonic waves in the kHz to MHz frequency spectrum are coupled into the construction element and/or detected and evaluated via permanently applied transducers which are usually piezoelectric. In this respect, different ultrasound parameters can be correlated with the strength properties of the materials as well as in particular with characteristic stages of the hardening process. In principle, measurements are possible at small, isolated material samples (usually small cubes), but also directly on the real construction element. Various processes have been described in the technical literature. They can roughly be divided into                reflection processes in which the reflected echoes reflected from the (accessible) surface of the construction element, from its rear wall or from components located in the construction element are evaluated;        transmission processes in which the waves passing through the construction element are evaluated;        Rayleigh wave processes which utilize the surface waves directed at the surface of the construction element;        resonance or impact echo processes in which natural resonances—in particular thickness resonances—of the examined construction element are measured; and        sound emission processes in which the sound emissions occurring during the hardening process in the construction element volume and generated by structural conversion and shrinking are detected.        
A plurality of material parameters (inter alia dependent on the frequency and the load) relevant to the material quality can be determined using these processes. In most cases, they are the sound velocity of pressure waves and shear waves (either directly via time of flight measurements or indirectly from amplitude reflection coefficients via the acoustic impedance) as well as from their damping constants. In addition, there are the corresponding Rayleigh wave parameters (sound velocity and damping) which are, however, directly linked with the elastic modulus of the pressure waves and shear waves.
None of the named processes is, however, currently able to determine the two parameters of sound velocity and damping for both types of waves and both for a reflection arrangement and a transmission arrangement (i.e. for surface testing and for volume testing). Usually only one wave mode is used (usually the pressure wave) and only surface properties or volume properties are determined. All arrangements can admittedly be combined with one another in principle, but additional (shear) transducers and measurement installations are required for this purpose. This increases the complexity and thus also the costs for the total structure, which is in particular a problem in the construction industry. If moreover two transducers are integrated in a measuring cell, two different routes have as a rule to be accepted for pressure waves and shear waves with a simultaneous measurement, which causes problems with heterogeneous media such as concrete since the material parameters scatter statistically along different routes.
The so-called prism technique is known from GB 2 215 056 A with which the measurement of sound velocities for pressure waves and shear waves can be realized with only a single (pressure wave) test head and for one and the same route. In this process, the material sample does not comprise a cube, as is the case for many standard concrete tests, but is rather a prism. By changing the angle of incidence of the pressure waves incident onto the prism base from a water bath, pressure waves and shear waves can be generated in a simple manner in the interior of the sample which are incident on the side surfaces of the prism in a perpendicular manner, are reflected there and subsequently return to the transducer on the same path. The special form of the sample allows the associated sound velocities to be determined only via the times of flight of reflected pressure waves and shear waves which are easy to measure and thus allows the Poisson number of the material to be determined via the known density of the pressure modulus and shear modulus or the elastic modulus.
The advantages of the prism method over traditional goniometer arrangements and over other processes for generating shear waves are described in detail therein.
However, only volume properties of the sample can be determined using the prism technique since only the waves passing through the material are evaluated. A second transducer would be necessary to determine surface properties by means of reflection at the prism base. The decisive disadvantage of the process is, however, that the critical angles for the optimum coupling of the two wave types during the hardening have to be constantly readjusted since they are material-dependent. Unavoidable slight deviations from the optimum angle thereby occur. These deviations admittedly do not have any direct effect on the time of flight measurement, but do noticeably influence the effectiveness of the sound coupling and thus also that of the damping measurement.
It must finally be mentioned that the functioning of the prism technique is based on the special geometry of the construction element to be examined. The process thus necessarily requires an isolated and especially manufactured prism-shaped material sample. A use on the real construction element made from concrete, e.g. by means of an attachment technique, is therefore not possible.